Simon Riggs wrote:
On Thu, 2007-04-19 at 22:37 +0200, Florian G. Pflug wrote:
The problem is that after a crash, the master might complete incomplete
actions via rm_cleanup() - but since it won't wal-log those changes,
the slave won't know about this. This will at least prevent the creation
of any further restart points on the slave (because safe_restartpoint)
will never return true again - it it might even cause data corruption,
if subsequent wal records are interpreted wrongly by the slave because
it sees other data than the master did when it generated them.


I agree the problem exists. It is somewhat rare because the idea is that
if the Primary crashes we would failover to the Standby, which would
mean that both Primary and Standby have executed rm_cleanup(), if
needed.

So in the case where the Primary fails and we choose *not* to failover,
there is a potential difficulty on the Standby.

It occured to me today that this might plague 8.1 too. As you explain below,
the problem is not really connected to restartpoints - failing to create
them is merely a sympton of this. On 8.1, this might still lead to rm_cleanup()
being called much "later" (if you consider the wal position to be the "time")
on the slave than on the master. I dunno if this really causes trouble - I
don't yet understand the btree code well enough to judge this.

The rationale for this fix could be described somewhat differently:

When we shutdown, we know for certain that safe_restartpoint() is true.
However, we don't know whether it is true because we successfully did a
clean shutdown, or because we crashed, recovered and then issued a
shutdown checkpoint following recovery. In the latter case we *must*
execute rm_cleanup() on the standby because it has been executed on the
primary. Not doing so at this point *might* be safe, but is not certain
to be safe. We don't *need* to log a restartpoint at this time, but it
seems sensible to do so.

While creating the patch, I've been thinking if it might be worthwile
to note that we just did recovery in the ShutdownCheckpoint
(or create a new checkpoint type RecoveryCheckpoint). This wouldl allow
for more error checking, because then the slave could check that
safe_restartpoint() is true for all ShutdownCheckpoints that were not
after recovering.

We need to check that rm_cleanup() routines don't assume that they will
only ever be called once or this will clearly fail. There is also no
need to call rm_cleanup() unless rm_safe_restartpoint() is false.

But a non-idempotent rm_cleanup() routine will cause trouble anyway,
if postgres crashes after having called rm_cleanup() but before creating
the ShutdownCheckpoint.

greetings, Florian Pflug


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